The present invention generally relates to sliding contact materials, sliding contact elements and sliding contact element producing methods, which are directed to achieving improved wear resistance under higher surface pressure conditions and the capability of noise prevention. More particularly, the invention relates to copper-base and/or iron-base sliding contact materials in which a Pb intermetallic compounds is dispersedly precipitated to improve sliding properties and relates to double layered sliding contact elements produced by sinter-bonding such sliding contact materials to an iron-base metal backing in an integral fashion thereby improving oil impregnation properties and lubricity. The invention also relates to economical methods for producing such double layered sliding contact elements. Hereinafter, sliding contact materials and sliding contact elements are simply referred to as xe2x80x9ccontact materialsxe2x80x9d and xe2x80x9ccontact elementsxe2x80x9d.
Bronze materials such as Cuxe2x80x94Snxe2x80x94Pb and lead-bronze materials are commonly used as materials for copper-base sintered bearings. Such materials are often integrally bonded to an iron-base metal backing to form a double layered sintered contact element such as a track roller provided in the base carrier of a construction machine.
As bearings used under high surface pressure and low speed conditions such as bushings for use in a construction machine, carburized or high-frequency-hardened wear-resistant steel bushings are widely known. When used in a construction machine, a bushing is subjected to high surface pressure and severe lubricating conditions and likely to generate undesirable abnormal noise during operation. To cope with this problem, there are often used high strength brass bushings or the above-described steel bushings which have undergone lubricating film coating treatment. High strength brass bushings are especially attractive because of their superior conformability. To prolong the intervals at which grease is fed to a bearing disposed in a construction machine etc., there has been proposed a graphite-embedded bearing material (e.g., 500SP produced by OILLESS INC.) that is produced by machining a high strength brass bushing to have holes and embedding porous graphite in the holes to be impregnated with oil. For the same purpose, a sintered metal body containing a large amount of solid lubricant (e.g., the SL alloys produced by TOSHIBA TUNGALOY CO., LTD.) is used.
Japanese Patent Publication (KOKOAI) No. 5-156388 (Japanese Patent Publication (KOKOKU) No. 56-12288) discloses a double layered sintered contact element for use under high surface pressure and its producing method. This contact element is formed by integrally bonding an aluminum-bronze-base sintered contact alloy, in which graphite is dispersed as a solid lubricating component within the range of 3 to 8 wt %, to a steel plate through a bonding layer.
Japanese Patent Publication (KOKAI) No. 3-232905 discloses a double layered sintered contact element having superior resistance to high load and impact. This element comprises (i) a metal backing that is made of a steel plate having a plurality of independent protrusions at its surface or steel plate having a series of connected protrusions and a plurality of independent recesses defined by the respective protrusions and (ii) a copper-base sintered alloy layer that is integrally formed with the metal backing so as to cover the protrusions of the surface and in which graphite is dispersed as a lubricating component in an amount of at least 3 wt %. This copper-base sintered alloy layer is composed of a low-density, high-oil-impregnating alloy region and a high-density, low-oil-impregnating alloy region.
Double layered sintered contact elements having a lead- bronze-base or bronze-base sintered contact layer are versatilely used as a sliding contact member of good conformability for the aforesaid lower track roller or engine metal, under low load, high speed and well lubricating conditions. However, they are inadequate for use as a bushing for a construction machine since they are easily worn out or fatigued due to lack of hardness for withstanding the high surface pressure and severe lubricating conditions.
When a steel bushing is used as a bushing in a construction machine, it is not fatigued but likely to cause seizure or unpleasant abnormal noise.
High strength brass contact materials produced by casting are substantially free from fatigue and generate little noise, but easily become short of lubricity. Therefore, they cannot completely prevent noise generation and are unavoidably susceptible to rapid wear when they are used in extremely low speed and high load applications such as when adapted in a construction machine.
Generally, in a high strength brass base contact material formed as a sinter contact alloy, the steam pressure of Zn contained is too high to carry out high density sintering. Therefore, sintering must be carried out under a pressurized condition when producing a double layered sintered contact element from such high strength brass contact material and, in consequence, production cost cannot be reduced.
In the case of steel bushings coated with a lubricating film, formation of a thick lubricating film is generally difficult and expensive and it is therefore apparent that after the lubricating film provided for such bushings is worn out, abnormal noise and seizure occur, like the case of ordinary steel bushings.
The graphite-embedded high strength brass bushing having more self-lubricity (e.g., 500SP produced by OILLESS INC.) requires a punching process for making a large number of holes for graphite embedding as well as a graphite filling process, in order to achieve high self-lubricity, which significantly increases cost. In many cases, the ratio of holes for graphite is normally restricted to 25 to 30% on a basis of area in view of cost, and therefore they cannot provide satisfactory self-lubricity over a long period of time.
The degree of sintering is a problem for a sintered metal body containing a large amount of known solid lubricant such as graphite, BN and MoS2 and therefore a pressure sintering means such as a hot press is required in order to achieve high density, resulting in increased cost. In addition, since the conventionally used solid lubricant is extremely soft, even a high-density metal sintered body suffers from high brittleness.
In the case of the double layered sintered contact element comprising (i) a steel plate serving as a metal backing and having a plurality of independent protrusions or a series of connected protrusions on its surface and (ii) an integral copper-base sintered contact alloy layer containing at least 3 wt % graphite, the steel plate backing formed into a specified shape is expensive and it is extremely difficult and costly to manufacture a metal backing which has protrusions (high-density portions) for sustaining a bearing load, in an area ratio of 30% or more.
Additionally, when bending the above material to produce a tubular bushing, bending stress is concentrated on the protrusions of the steel plate which are harder than other areas, so that exfoliation is likely to occur in the joint interface between the steel plate and the copper-base sintered contact material and the deformation resistance occurring in the bending operation is unevenly distributed in the steel plate. This makes it difficult to uniformly bend the material into a tubular shape. In consequence, a lot of machining work is involved in producing a tubular bushing, and the cost for inspecting the quality of bonding between the contact material and metal backing of the final product increases.
The invention is directed to overcoming the foregoing problems. Therefore, an object of the invention is to provide copper-base and/or iron-base contact materials in which Pb intermetallic compounds are dispersedly precipitated to achieve highly improved sliding properties. Another object of the invention is to provide double layered contact elements and their economical producing methods, the contact elements being produced by sinter-bonding the above contact materials to an iron-base metal backing in an integral fashion and being improved in oil impregnation and lubricity.
To achieve the above objects, the invention includes the following technical means.
According to a first aspect of the invention, there are provided copper-base contact materials and copper-base and/or iron-base sinter contact materials,
which are softer and more conformable than the above-described conventional steel bushing material, and
which are basically Cuxe2x80x94Pbxe2x80x94Ti alloys and xe2x80x94Mg alloys having high seizure and wear resistance like high strength brass material, which alloys contain a Pb intermetallic compound dispersedly precipitated therein by virtue of the presence of Ti or Mg.
In contrast with the conventional lead-bronze base contact materials and iron-base sinter contact materials in which Pb is dispersedly precipitated simply as a metal component, the contact material of the invention contains Pb dispersed in the form of a Pb intermetallic compound. With this arrangement, the sliding properties of the contact material under high surface pressure and severe lubricating conditions can be highly improved. Since Pb is dispersedly precipitated in the form of a Pb intermetallic compound, the contact material may contain Pb at least in the range of from 1 to 30 wt %. This Pb range is wider compared to the case where Pb is precipitated as a metal.
As counter metallic elements which strongly combine with Pb to form an intermetallic compound, 1A, 2A, 3A, 4A and 6B groups in the periodic table shown in FIG. 1, lanthanides and actinides may be used in combination, but Ti, Mg, Ca, Ba, Zr, Li, Hf, La, Te, Se, Sm are preferable in view of cost and availability. Besides, Ti, Mg and Zr are more preferable for the above reason. The same effect can be expected when other elements than the above are used for forming a Pb intermetallic compound.
Although the contents of the above metallic elements can be calculated from the molar ratio of a Pb intermetallic compound such as CaPb3, they needs to be contained in amounts of substantially 0.5 wt % or more.
Accordingly, the bronze-base sinter contact material is especially important, which is designed to contain Ti and Mg in an amount of 0.5 to 10 wt % in order to improve strength, sliding properties and cost performance and to further contain Sn in an amount of 1 to 10 wt % in order to ensure bonding strength relative to a metal backing and facilitate sintering operation.
The upper limit of the amount of Ti and Mg is preferably 10 wt % in view of cost.
Since it is known that the amount of a Pb intermetallic compound which contributes to seizure resistance is about 0.5% by volume in the case of conventional contact materials containing dispersed hard particles, the amount of Pb is preferably 1 wt % or more and the upper limit of Pb is preferably 30 wt % in view of strength. Taking the environmental problems imposed by Pb in the sintering process into account, the amount of Pb should be restricted to 15 wt % or less. Further, copper-base contact materials containing Ti are known to react to precipitate other Ti intermetallic compounds than the intermetallic compounds of Ti and Pb and undergo age hardening. It is also known that the strength and hardness of Ti-containing copper-base contact materials can be improved by adding alloy elements such as Al, Ni, Si, Fe, Mn, Cr, Be and the like. In view of these facts, it is appropriate to contain Ti within the range of up to 5 wt % in order not to impair sintering properties.
It is apparent that the addition of the above metallic elements having the ability of forming a Pb intermetallic compound enables fining of Cu-base sinter alloy structures and in consequence the uniform dispersion of Pb, providing excellent sliding properties.
The appropriate amount of Ti is 0.5 wt % or more by which a Ti intermetallic compound precipitates and the upper limit of the amount of Ti is preferably 10 wt % in view of cost. For producing a construction machine bushing or the like for use under high surface pressures, the more preferable amount of Ti is 2 wt % or more, taking the hardness (Hv=150 or more according to the actual record) of high strength brass bushings into account.
In conventional contact materials containing dispersed hard particles, remarkably stable sliding properties can be admitted when 0.5% by volume of hard particles are dispersed. Therefore, the amount of Pb is 1 wt % or more and, more preferably, 5 to 10 wt % when converted to a Pb intermetallic compound basis.
It is obvious that the same inventive factors as those of the above copper-base sinter contact materials can be achieved by cast materials. Accordingly, the scope of the invention covers the case where a cast material is machined and formed into the copper-base contact element having improved sliding properties.
According to the invention, the copper-base contact materials prepared by casting and the copper-base and/or iron-base sintered contact materials prepared by sintering may be designed, in consideration of the surface pressure imposed on these materials during operation and the intervals at which grease is supplied, to have 10 to 70% holes on an area ratio basis and contain grease or oil impregnated plastics embedded in the holes. The invention also covers the case where a copper-base sinter material powder which is expandable by sintering is introduced in the holes of the copper-base and/or iron-base contact materials and sintering is carried out to form a porous sintered body excellent in oil impregnation so as to be bonded to the contact materials, as disclosed in Japanese Patent Publication (KOKAI) No. 8-291306.
According to a second aspect of the invention, there is provided a double layered sintered contact element whose outermost sliding surface is made of the above-described copper-base sinter contact material. This reduces the use of the copper-base sinter contact material thereby saving costs. The double layered sintered contact element contains at least 1 wt % or more Sn in order to ensure bonding strength between the contact material and an iron-base metal backing. In order to lower sintering temperature, the preferable amount of Sn should not exceed 10 wt %.
The addition of P causes the generation of a liquid phase at low temperatures thereby accelerating sintering. To restrict excessive reaction with the iron-base metal backing, the amount of P is preferably adjusted so as not to exceed 1 wt %.
For bonding to the iron-base metal backing, a powder of the above copper-base sinter alloy is sprayed onto the surface of the iron-base metal backing and sinter bonding is then carried out in a neural or reducing atmosphere at at least not higher than 890xc2x0 C. which is the eutectic temperature of Cuxe2x80x94Ti duel alloys. Thereafter, a pressurizing process such as rolling is carried out to form a copper-base sintered layer having desired density or thickness. Bending may be carried out at the same time. Then, the material is again sintered in the same atmosphere described above. The reason why the temperature of the first sintering operation is restricted to the eutectic temperature (890xc2x0 C.) of Cuxe2x80x94Ti duel alloys or less is that a Ti compound precipitating at the joint interface between the contact material and the iron-base metal backing is restricted to thereby restrain the occurrence of exfoliation at the joint interface during the subsequent process of bending. Where the bending process is incorporated, the temperature of the second sintering operation is preferably 950xc2x0 C. or less. Where the pressurizing process for density adjustment is carried out without the bending process, the preferable second sintering temperature is 890xc2x0 C. or less and it is preferable to carry out sintering again at the target temperature of 950xc2x0 C. or less after the second sintering operation.
Where a compact sheet which has undergone rolling or press molding is set on the iron-base metal backing for sintering, it is preferable that after sintering at a temperature of 890xc2x0 C. or less, the composite material be bent and sintered again. The reason for this is that the precipitation of a Ti compound at the joint interface is restricted to prevent exfoliation as stated earlier.
According to a third aspect of the invention, when forming a double layered sintered contact element by integrally bonding the copper-base contact material containing Ti and Pb to the iron-base metal backing, a sintered insert layer may be interposed between them. With the provision of the sintered insert layer, the precipitation of the aforesaid compound at the joint interface can be further prevented in the bending process thereby increasing plastic deformability. As a result, the number of sintering operations can be reduced to increase productivity. In addition, not only improved bonding quality but also cost savings can be achieved.
The copper-base contact material used in the above double layered sintered contact element having the sintered insert layer may be a cast material or sinter contact material. As described earlier, it is apparent that a copper-base contact element having 10 to 70% holes on an area ratio basis can be produced at low cost. Additionally, the contact material, which has holes containing the above expansive copper-base sinter material sinter-bonded thereto, may be integrally adhered to the metal backing with the sintered insert layer between. This enables an inexpensive, double layered sinter-bonded contact element.
According to a forth aspect of the invention, the above-described sintered insert layer may be formed from an iron-base sinter material, thereby markedly reducing the cost of the material of the sintered insert layer. A preferable iron alloy used for making the sintered insert layer contains (i) an iron or iron alloy powder made by the reduction and/or atomizing method commonly used in ordinary metallurgy, as a major component; (ii) at least 20 to 60 wt % copper; and (iii) 2 to 7 wt % Sn. The iron alloy for the sintered insert layer preferably contains a component which generates a liquid phase having a lower-melting point such as Pb and P.
It should be noted that when the iron alloy for the sintered insert layer is sprayed onto the metal backing and then sinter bonding is carried out in the temperature range of 890xc2x0 C. or less, not significant compaction but a slight amount of expansion is admitted and the structure after sintering has many voids formed by linked iron-base powder particles, resulting in a low apparent density. Where a bronze or phosphor-bronze sinter alloy is used for the sintered insert layer, the degree of sintering rapidly increases from 800xc2x0 C. or more and the alloy is melted at an adequate sintering temperature for the copper-base sinter contact alloy of the invention containing Ti and a large amount of Pb, or the copper-base sinter contact material is significantly alloyed with the insert layer material so that the quality of the sliding contact layer cannot be controlled. For this reason, the above iron-base sinter material is used for the insert layer of the invention.
In addition, since the surface of the iron-base sintered insert layer is rough, when spraying a mixed powder of the copper-base sinter contact material onto the iron-base insert layer, spraying can be easily controlled so as to vary the thickness of the contact material layer. Where the thickness of the contact material layer is less than 0.5 mm, rolling can be directly performed after the spraying operation and therefore sintering can be advantageously successively carried out in the next process. This contributes to cost savings in the production of the double layered sintered contact element. Where the above thickness is 0.5 mm or more, such direct rolling after the spraying operation cannot be carried out and therefore it is preferable to carry out a pressing operation such as rolling during re-sintering to adjust the density and thickness of the material.
In the case of the double layered sintered contact element produced in the way described above, since the deformation resistance of the copper-base sinter contact material is much smaller than that of the iron-base insert layer material, the surface contact layer has high density and strength while the iron-base sintered insert layer can be made porous so that the resultant double layered sintered contact element has superior oil impregnation properties.
According to a fifth aspect of the invention, the porous sintered insert layer, which has been adjusted in thickness and bonded to a steel plate, is machined so as to be rugged and after the copper-base contact material is sprayed to the machined surface, rolling and re-sintering are carried out, whereby a low-cost double layered sintered contact element having low-density, high-oil-impregnation regions and high-density, low-oil-impregnation regions at the sintered contact layer can be obtained.
This material is intended for further enhancing the advantageous feature (i.e., porosity) of the above iron-base sintered insert layer. By machining the iron-base sintered insert layer to have convex portions and concave portions and spraying a mixed powder of the sinter contact material onto the rugged surface, the independent or linked concave portions are filled with a comparatively large amount of the contact material powder while the convex portions are slightly covered with the contact material powder. Then, rolling for obtaining a specified density or thickness is successively carried out like the above case and sintering is then carried out at 950xc2x0 C. or less so that the double layered sintered contact element can be easily produced. This process for producing the double layered sintered contact element may be alternatively arranged such that, after a powder of the sintered contact material is sprayed onto the rugged surface, sintering is once carried out at 890xc2x0 C. or less and followed by rolling for density and/or thickness adjustment and thereafter sintering is again carried out.
According to a six aspect of the invention, a mixed powder of the above copper-base and/or iron-base sinter contact material is compacted within a die under a specified pressure to form a tubular green compact; this green compact is inserted into a tubular iron-base metal backing; and sinter bonding is carried out to integrally form a double layered sintered contact element.
As described above, the inventors have developed a copper-base sinter contact material in which a Pb intermetallic compound is dispersed in the base and which contains Ti and Mg, thereby achieving a sinter contact material which has superior seizure resistance and more ability for withstanding wearing sliding conditions under high surface pressure, compared to the conventional high strength brass contact material and iron-base contact material, and which, further, has the capability of restraining abnormal noise even under severe lubricating conditions. The inventors also have developed a double layered sintered contact element formed from the above sinter contact material thereby achieving a low-cost sliding contact element having the aforesaid excellent features. Further, the inventors propose the provision of an iron-base insert layer between the above-described sinter contact material and an iron-base metal backing as well as the provision of low-density high-oil-impregnation copper-base sintered contact regions and high-density low-oil-impregnation copper-base sintered contact regions on the iron-base insert layer by making the surface of the insert layer rugged. With this arrangement, oil content can be increased and feeding of a lubricant to the high-density low-oil-impregnation copper-base sintered contact regions subjected to more severe lubricating conditions is facilitated to thereby achieve a highly seizure-resistant, low-cost double layered sintered contact element.